Process for extruding and quenching a polymeric film

ABSTRACT

Quenching molten polymeric films, especially polyester films, on casting surfaces by means of a combination of electrostatic and suction pinning.

The present invention relates to a process and apparatus for quenchingextruded polymeric films.

Polymeric films are commonly made by extrusion from a slot die on to arotating cooled casting drum where they are solidified into a formsuitable for subsequent film making operations. In the case of certaincrystallisable polymeric materials, such as polyethylene terephthalate,it is important to rapidly quench the extruded film to the amorphousstate in order to minimise the onset of excessive crystallinity in thefilm which would cause embrittlement and interfere with subsequent filmmaking operations. One method of achieving such quenching is to ensurethat intimate contact is established between the film and the castingdrum surface by depositing electrostatic charges upon the molten filmbefore it reaches the casting drum and usually electrically earthing thecasting drum. The electrostatic charges are attracted to the castingdrum and establish the desired contact. Such a method is often termed"electrostatic pinning" and is described in British pat. specificationNo. 911,528, the disclosure of which is incorporated herein byreference.

Electrostatic pinning has been used successfully in the production oflinear polyester films under casting conditions and output ratesavailable heretofore. However, at higher casting speeds, i.e. the speedof extrusion and rotation of the casting drum, which become ofimportance for economy of production, electrostatic pinning does notfunction quite so satisfactorily. One disadvantage is that bubblesbecome entrapped between the film and the casting drum surface with theresult that thermal contact between the film and the casting drumsurface is impaired thereby reducing the casting efficiency. Theentrapped bubbles also tend to cause irregularities in the surface ofthe quenched film. Several proposals for casting at high speed inassociation with electrostatic pinning have been disclosed.

British pat. specification No. 1,310,604 discloses a process of highspeed casting in which the electrostatic pinning is restricted to theedges of the molten film as it is deposited on a casting drum and an aircurrent is applied across the complete film width to urge the film intocontact with the casting drum. Further aspects of apparatus suitable forelectrostatically pinning film edges are disclosed in U.S. pat.specification Nos. 3,758,251 and 3,779,682.

British patent specification 1 309 664 relates to an electrostaticpinning apparatus in which the electrical field producing the pinningcharges is established between two electrodes, one maintained at a highvoltage potential and the other being electrically earthed.

British pat. specification No. 1,309,449 describes another form ofelectrostatic pinning apparatus in which the pinning electrode issurrounded by a gas and the pinning conditions employed are such that nospark breakdown occurs.

Another proposal for the avoidance of entrapped bubbles appears inBritish pat. specification No. 1,302,642 which discloses that thecasting drum surface should be rough and have interconnecting pits.

In a further proposal, disclosed in British pat. specification No.1,333,133, a fluid bearing is positioned between the extrusion die andthe touchdown of the film on to the casting drum to displace the filmfrom its normal catenary path to the path which is nearer toperpendicular to the casting drum surface.

The above proposals involve complex modifications to the casting andpinning apparatus and the conditions for its operation.

Other means of casting polymeric films on to casting surfaces whichoperate without the use of electrostatic pinning have been disclosed inthe art. British pat. specification Nos. 946,097 and 1,066,493 disclosethat a vacuum should be applied to a sealed chamber located between themolten film and the casting surface.

None of the prior art references mentioned above relates to theessential combination of electrostatic pinning and suction taught by thepresent application.

According to the present invention a process for extruding and quenchinga polymeric film comprises extruding a molten polymeric film on to anelectrically earthed moving casting surface, said casting surface movingat a linear speed greater than the speed at which bubbles can beprevented from being entrapped between the molten polymeric film and thecasting surface when electrostatic pinning is employed alone, depositingelectrostatic charges upon part or the whole of the molten film width bymeans of an electrostatic pinning electrode supported in space relationto the molten film in the proximity of or prior to the region of firstcontact of the film with the casting surface, and applying suction tothe space bounded by the path of the molten film prior to its firstcontact with the casting surface and that part of the moving castingsurface prior to the region of first contact with the molten film, thedeposited electrostatic charges and the suction operating in combinationto adhere the molten film to the casting surface.

The invention also relates to an apparatus for extruding and quenching apolymeric film, which comprises an electrically earthed movable castingsurface arranged to receive the extruded polymeric film, anelectrostatic pinning electrode supported in spaced relation to the pathof the molten film in the proximity of or prior to the region of firstcontact of the film with the casting surface, and means for applyingsuction to the space bounded by the path of the molten film prior to itsfirst contact with the casting surface and that part of the castingsurface prior to the region of first contact with the molten film.

The present invention may be used for the quenching of all polymericmaterials which are capable of being formed in a flat self-supportingfilm by extrusion and quenching, e.g. polystyrene, polyamides, polymersand copolymers of vinyl chloride, polycarbonate, polymers and copolymersof olefines such as polypropylene, and polyesters of dibasic aromaticcarboxylic acids with divalent alcohols. Polyester films which may bequenched according to this invention may be produced by condensing oneor more dicarboxylic acids or their lower alkyl diesters, e.g.terephthalic acid, isophthalic acid, phthalic acid, 2,5-, 2,6- and2,7-naphthalene dicarboxylic acid, succinic acid, sebacic acid, adipicacid, azelaic acid, diphenyl dicarboxylic acid, andhexahydroterephthalic acid, or bis-p-carboxyl phenoxy ethane, optionallywith a monocarboxylic acid, such as pivalic acid, with one or morealkylene glycols, e.g. ethylene glycol, 1,3-propanediol, 1,4-butanediol,neopentyl glycol and 1,4-cyclohexane-dimethanol. The invention isparticularly suitable for the quenching of molten polyethyleneterephthalate films. The films quenched according to this invention maycontain conventional additives such as particulate fillers added, forexample, for slip or surface matting properties, anti-static agents,dyes and pigments. The invention is also suitable for the quenching ofcomposite films comprising two or more polymeric layers.

The moving casting surface may take any convenient form such as a movingmetal band or a rotating cylindrical metal drum, the latter beingpreferred. The casting surface may be highly polished to enable thequenched film to be separated and stripped from it and may be cooled tothe appropriate quenching temperature by means of cooling fluids. In thecase of a casting drum, the cooling fluid, e.g. water, may be circulatedthrough the drum internally.

The combination, according to this invention, of electrostatic pinningand suction is essential to satisfactory quenching with the avoidance ofentrapped bubbles at higher casting surface speeds, i.e. at speedsgreater than the speed at which bubbles can be prevented from beingentrapped when electrostatic pinning alone is employed. The speed atwhich bubbles cannot be prevented from being entrapped depends on thecharacteristics of the quenching apparatus and the conditions employedfor electrostatic pinning. Thus, for casting surfaces in the form of acylindrical drum, the speed at which bubbles cannot be preventeddepends, inter alia, on the diameter of the drum and the incidence ofbubbles occurs at slower peripheral drum speeds as the diameter of thedrum increases. Accordingly, the combination of electrostatic pinningand suction becomes essential at slower drum speeds for larger diameterdrums than for smaller diameter drums. For a given casting surface, theactual speed at which bubbles cannot be prevented from being entrappedusing electrostatic pinning alone is influenced by the setting of theapparatus and especially the pinning voltage and the positioning of thepinning electrode relative to the molten film. Such adjustment of theelectrostatic pinning equipment is well understood in the art and thegreatest speed at which a given apparatus can be operated satisfactorilywithout the occurence of pinner bubbles can be readily determined by theskilled man in the art.

For most casting assemblies employing rotating casting drums, the speedat which the combination of electrostatic pinning and suction becomesessential will exceed 50 feet/minute and generally 100 feet/minute. Fortypical casting apparatus, the inventive combination will becomeessential at speeds exceeding about 150 feet/minute.

According to this invention the electrostatic pinning charges may beapplied across part or the whole of the film width. When applied to onlypart of the film width, the charges are preferably applied to the edgemargins of the film by means of suitably located pinning electrodes ateach edge of the film. The electrodes in such an arrangement mayconsists of one or more point probes or wire electrodes suitablyarranged or insulated that they only deposit charges on the edge marginsof the film. Alternatively and preferably, the electrostatic pinningelectrode is arranged to deposit electrostatic charges across the wholewidth of the molten film and is preferably in the form of a fine wireextending transversely across the casting surface, although sharpelectrodes of other form, e.g. knife edges or a multiplicity ofsharpened points, may be employed if desired.

Preferably, the electrostatic pinning electrode is located so that theelectrostatic charges are deposited on the side of the molten film otherthan the side which comes into contact with the casting surface.However, if desired, it is possible to locate the electrostatic pinningelectrode to deposit charges on the side of the molten film which doescome into contact with the casting surface.

When a wire pinning electrode is employed, the wire may have a diameterof up to 0.030 inch, and may for example have a diameter of about 0.007inch. The wire may be a stainless steel or tungsten wire and locatedfrom 0.1 to 0.5 inch from the molten film.

The conditions employed for electrostatic pinning may be thoseconventionally employed, e.g. as described in British pat. specificationNo. 911,528. Thus, the electrostatic pinning electrode may be maintainedat a high potential relative to the earthed casting surface byconnection to a direct current voltage supply, the other terminal of thesupply also being earthed. A suitable supply provides a voltage in therange 2 to 30 kilovolts and a current of 0.3 to 50 milliamperes,preferably up to 5 milliamperes. A typical supply comprises a voltage ofabout 5 kilovolts and a current of about 2 milliamperes.

Alternatively, the pinning electrode may be connected to an alternatingsupply, as disclosed in German application No. 2,405,863, comprising avoltage of symmetrical or asymmetrical waveform exceeding 4.0 kilovoltspeak to peak and having a frequency not exceeding that at which positiveand negative electrostatic charges emitted by the electrode are unableto cross the space between the electrode and the molten film before thepolarity of the voltage on the electrode charges.

The suction or partial vacuum may be established in an enclosed chamberformed by the molten film and the part of the moving casting surfacepreceding the region of first contact of the film with the castingsurface and an enclosing housing which may be connected to a suitablesuction pump. However, it has been found that a chamber is not essentialto the operation of this invention and that a suitably located suctionknife is adequate to establish the desired level of suction or partialvacuum. Such a suction knife may have a slot orifice measuring from 0.06to 1.5 inch in its narrower dimension and extending substantially acrossthe width of the film or beyond the width of the film and have a housingcommunicating with a suction pump. Preferably, the slot orifice islocated between 0.06 and 3.5 inches from the molten web and between 0.06and 3.0 inches from the casting surface so as to establish a suction orpartial vacuum in the space between the molten film and the castingsurface. A suction or partial vacuum in the region of 0.1 to 3.0 inchesof water has been found sufficient to achieve satisfactory castingwhilst avoiding bubbles being entrapped between the molten film and thecasting surface. Preferably, the suction or partial vacuum may be up to1.5 inch, preferably up to 1.0 inch and most preferably in the range 0.2to 0.6 inch water.

According to one embodiment of the invention, the suction or partialvacuum may be applied solely in the space bounded by the moving castingsurface and the molten film, e.g. by locating the slot orifice of asuction knife in the space. It has been observed that air may be drawnaround the edges of the film into the suction or partial vacuum. Thepassage of the air across the edges of the film may become non-uniformand turbulent thus creating non-uniform cooling of the edge regions ofthe molten film. It has been found in some processes that this localisedcooling may lead to edge imperfections and irregularities in the film.For example, such imperfections and irregularities may be exhibited bypolyester films such as polyethylene terephthalate films quenched inthis way, especially if the film is subsequently stretchedlongitudinally.

Such imperfections and irregularities may be reduced in anotherembodiment of this invention, in which means are provided for applying asuction or a partial vacuum substantially in the vicinity of the moltenfilm edges and substantially transversely to the direction of movementof the casting surface. The transverse suction or partial vacuum may beapplied by means of suction knives located in the vicinity of the moltenfilm edges. The transverse suction or partial vacuum may equal or differfrom that applied to the space between the moving casting surface andthe molten film. Preferably, the transverse suction or partial vacuum isgreater in magnitude than that applied between the moving castingsurface and the molten film, which may have the magnitude hereinbeforespecified, and in the range 1.5 to 5 inches of water.

The suction or partial vacuum may have a cooling effect upon theextrusion die as the result of the passage of air across the die and mayresult in local freezing of the molten polymer in the die. This problemmay be avoided by shielding or thermally insulating those parts of thedie which would otherwise be exposed to the suction or partial vacuum.

The use, according to this invention, or electrostatic pinning incombination with suction or partial vacuum facilitates the satisfactorycooling and quenching of molten polymeric films without the need for thecomplex casting conditions and apparatus disclosed in the prior artreferences mentioned above which are concerned with high speed casting.The avoidance of entrapped bubbles enables the casting surface tooperate at a high speed in the production of cast film of satisfactoryquality. This is especially useful in the production of polyester films,such as polyethylene terephthalate films, at higher speeds.

In the absence of a suction or partial vacuum, the motion of the castingsurface tends to drag the molten film away from the shortest pathspanning the gap between the extrusion die and the casting surface. In atypical process, the extrusion die is arranged to extrude the moltenfilm vertically and downwardly towards the casting surface, and athigher speeds the molten film follows a catenary path to the castingsurface which is longer than the direct or shortest path to the castingsurface. The reduction in film width, often termed "neck-in," increasesas the path of the unsupported molten film increases. The application ofa suction according to this invention serves to maintain the molten filmapproximately in the direct or shortest path thereby reducing the effectof neck-in.

An additional advantage in restraining the molten film approximately tothe shortest path is that imperfections which might be imposed on thefilm surface by the die lips when the film is dragged out of the normaldirection of extrusion at higher casting speeds are also reduced.

The cast films produced according to this invention may be subjected toany additional film-forming operations which are known in the art, suchas stretching in one or more directions to effect molecular orientationfollowed optionally by heat setting. Such operations are disclosed forpolyethylene terephthalate films in British pat. specification No.838,708.

The invention is now further described with reference to theaccompanying drawings, in which:

FIG. 1 is a perspective view of one embodiment omitting the extrusiondie for simplicity;

FIG. 2 is a cross-sectional view of the apparatus illustrated in FIG. 1;

FIG. 3 is a cross-sectional view of an apparatus similar to that shownin FIGS. 1 and 2 but omitting the suction knife;

FIG. 4 is a cross-sectional view similar to FIG. 2 but on an enlargedscale and omitting the electrostatic pinning wire and the suction knife;

FIG. 5 is a cross-sectional view of an alternative embodiment;

FIG. 6 is a perspective view of a suction knife having provision fortransverse suction; and

FIG. 7 is a perspective view of an apparatus incorporating a suctionknife as illustrated in FIG. 6.

The apparatus illustrated in FIGS. 1 and 2 is suitable for the castingof molten polyester films such as polyethylene terephthalate films, inrelation to which the apparatus is described. The molten polyethyleneterephthalate film 4 is extruded in a conventional manner through a filmforming slot die 1, which is illustrated in FIG. 2, on to a conventionalwater-cooled steel casting drum 2 which is electrically earthed and hasa polished casting surface 3. In operation, the casting drum 2 isrotated in the direction of the arrow A with peripheral speeds in therange 140 to 240 feet/minute so that the molten film 4 is laidcontinuously upon the casting surface 3 as it emerges from the die 1.After quenching, the film is stripped from the casting surface 3 bypassage of a roller 5.

An electrostatic pinning wire electrode 7 extends transversely acrossthe molten film and parallel to the axis of the casting drum 2. As shownin FIG. 1, tubular sleeve insulators 8 made of polytetrafluoroethyleneelectrically insulate the pinning wire 7 over the parts of the castingsurface 3 which remain exposed beyond the edges of the cast film toprevent sparking between the pinning wire 7 and the casting surface 3when the apparatus is in operation. The pinning wire 7 is located in theproximity of the region of first contact of the molten film 4 with thecasting surface 3 and is connected to one terminal of a direct currentpower supply (not shown in the drawing) the other terminal beingconnected to earth. The power supply has an output of up to 20 kilovoltsand up to 5 milliamperes such that electrostatic pinning charges areemitted and deposited upon the molten film 4 thereby contributing to itsadhesion to the casting surface 3.

A suction knife 10 is located on the opposite side of the molten film 4from the electrostatic pinning wire 7 and consists of a housing 11having a slot orifice 12 which extends across the whole width of themolten film and a narrower dimension of 0.125 inch. The housing 11communicates through a duct 14 with a suction pump (not shown in thedrawing) by which a suction of up to 3 inches of water gauge can beapplied to the housing. The slot orifice 12 of the suction knife 10 islocated so as to apply a suction in the space bounded by the path of themolten film 4 between the die 1 and the moving casting surface 3 and thepart 15 of the casting surface 3 prior to the region of first contact ofthe film with the casting surface 3. The slot orifice 12 is locatedapproximately 2 inches from the molten film 4 and 1 inch from thecasting surface 3.

FIG. 3 illustrates the effect of operating the apparatus shown in FIGS.1 and 2 without the suction knife 10. It is seen that, with bothelectrostatic pinning and partial vacuum in operation, as shown in FIG.2, the molten film traverses approximately the shortest path between thedie 1 and the casting surface 3, i.e. a path which is approximatelyperpendicular to the casting surface 3. When the suction knife 10 is notemployed, as in FIG. 3, the molten film traverses a longer catenary path17. As a result, the electrostatic pinning wire 7 must be located in adifferent position. The longer catenary path 17 imposes a greatertransverse neck-in on the film than can arise when the film traversesthe shorter path shown in FIG. 2. In addition, defects can be imposed onthe film surface when casting is effected as shown in FIG. 3 especiallyby the die lip 18 which bears against the film surface.

FIG. 4 illustrates the spacing between the extrusion die and the firstcontact of the molten film 4 with the casting drum 2. This spacing,designated a, corresponds to the distance, measured perpendicular to thecentral plane through the extrusion die, to the first contact of themolten film 4 with the casting drum 2.

FIG. 5 illustrates an apparatus, suitable for the casting of moltenpolyester films such as polyethylene terephthalate films, which has analternative construction to that shown in FIG. 1. The apparatuscomprises a film-forming slot die 21 and a water-cooled steel castingdrum 22 having a polished casting surface 23. The casting drum 22 iselectrically earthed to facilitate the electrostatic pinning of themolten film to the casting surface 23 by means of an electrostaticpinning wire electrode 24. Molten polymer 25 extruded through the die 21is fixed to the casting surface 23 by the combined effect ofelectrostatic pinning and suction through a suction knife 26, theorifice 27 of which is located in the space bounded by the path of themolten polymer between the die 21 and the casting surface 23.

The suction knife illustrated in FIG. 6 has a main slot orifice 29 andtwo side orifices 30 and 31.

The apparatus illustrated in FIG. 7 comprises a suction knife 33 of theform illustrated in FIG. 6. A molten film of polyester 34, such aspolyethylene terephthalate, is cast on to a water-cooled steel castingdrum 35 having a polished casting surface. An electrostatic pinning wireelectrode 36 is insulated at its ends by tubular sleeve insulators 37made of polytetrafluoroethylene. The suction knife 33 is located so thatthe main slot orifice 38 applies a suction or a partial vacuum in thedirection of rotation of the casting drum 35 and the side orifices 39and 40 are positioned close to the edges of the molten film andestablish a suction or partial vacuum in the vicinity of the film edges.

The invention is further illustrated by the following comparativeexamples and examples of the invention.

COMPARATIVE EXAMPLE A

Molten polyethylene terephthalate film was cast and quenched using theapparatus illustrated in FIG. 1 of the drawings. The casting drum had aradius r. The power supply was disconnected from the pinning wire 7 sothat casting was effected solely under the influence of the suctionimposed by the suction knife 10. The suction applied was 0.5 inch watergauge. At a casting drum peripheral speed of 100 feet/minute, finemachine direction corrugations were observed in the quenched film andwere attributed to lateral slippage of the film on the casting drum. Thecorrugation defects were also encountered at other drum speeds. Thequenched film was unsuitable for the production of commerciallyacceptable oriented and heat set film.

COMPARATIVE EXAMPLES B TO E

Molten polyethylene terephthalate film was cast and quenched using thesame apparatus used in Comparative Example A and as illustrated in FIG.1 of the drawings. The casting drum had a radius r. The suction knife 10was disconnected from the suction pump so that casting was effectedsolely under the influence of electrostatic pinning. The incidence ofentrapped bubbles under various conditions of production are noted inthe following Table 1.

                                      TABLE 1                                     __________________________________________________________________________                                      Die to                                                                 Extrusion die                                                                        contact gap                                 Com- Casting Pinning wire                                                                         Pinning wire                                                                         to casting                                                                           a in                                        parative                                                                           drum speed                                                                            potential                                                                            current                                                                              drum gap                                                                             FIG. 4                                                                              Neck-in                                                                            Pinner                           Example                                                                            feet/minute                                                                           kilovolts                                                                            milliamperes                                                                         inch   inch  inches                                                                             bubbles                          __________________________________________________________________________    B    150     6.0    1.45   0.44   1.0   --   None                             C    180     6.0    1.5    0.44   1.0   --   Fine                                                                           bubbles                         D    200     6.0    1.6    0.44   1.25  2.2  Moderate                                                                      bubbles                          E    230     6.0    1.7    0.44   1.5   2.2  Excessive                                                                     bubbling                         __________________________________________________________________________

It is seen from Table 1 that although no bubbles were entrapped at acasting drum speed of 150 feet/minute, fine bubbles were encountered at180 feet/minute and bubbling became worse as the drum speed wasincreased. There was also substantial neck-in. The film produced underconditions which resulted in pinner bubbles exhibited surfaceimperfections resulting from the bubbles and was not suitable for theproduction of oriented end heat set film.

EXAMPLES 1 AND 2

Molten polyethylene terephthalate film was cast and quenched accordingto the invention with both electrostatic pinning and suction using theapparatus illustrated in FIG. 1 of the drawings. The casting drum had aradius r and the slot die 1 had a length the same as that used inComparative Examples B to E.

The conditions of casting and quenching and the resulting filmcharacteristics are shown in Table 2.

                                      TABLE 2                                     __________________________________________________________________________                                         Die to                                                            Suction                                                                            Extrusion die                                                                        contact gap                                   Casting                                                                             Pinning wire                                                                         Pinning wire                                                                         inch of                                                                            to casting                                                                           a in                                          drum speed                                                                          potential                                                                            current                                                                              water                                                                              drum gap                                                                             FIG. 4                                                                              Neck-in                                                                            Pinner                        Example                                                                            feet/minute                                                                         kilovolts                                                                            milliamperes                                                                         gauge                                                                              inch   inch  inches                                                                             bubbles                       __________________________________________________________________________    1    180   6      2.4    0.5  0.44   0.38  1.30 None                          2    230   5      2.3    0.5  0.44   0.44  1.41 None                          __________________________________________________________________________

The film produced was of good quality. No pinner bubbles were entrappedbetween the molten film and the casting drum surface. The neck-in wasless than that achieved in Comparative Examples D and E, the reductionbeing partly due to the shorter path of the molten film from the die tothe casting drum (as illustrated by FIGS. 2 and 3) and partly becausethe die could be located closer to the quenching drum as a result of thestability imposed on the molten film by the suction.

The quenched film was subjected to molecular orientation and heatsetting by conventional techniques to produce film of good quality.

COMPARATIVE EXAMPLE F AND EXAMPLES 3, 4 AND 5

Molten polyethylene terephthalate film was cast and quenched byelectrostatic and suction means using the apparatus shown in FIG. 5 ofthe drawings. The die lip adjacent the suction knife 26 was thermallyinsulated against the cooling effect of the suction knife by locating asheet of insulating material on the face of the die above the suctionknife. Comparative Example F employed no suction. The casting andquenching conditions and the characteristics of the resulting film areshown in Table 3.

                                      TABLE 3                                     __________________________________________________________________________                             Suction                                                                            Extrusion die                                        Casting                                                                             Pinning wire                                                                         Pinning wire                                                                         inch of                                                                            to casting                                           drum speed                                                                          potential                                                                            current                                                                              water                                                                              drum gap                                                                             Neck-in                                                                            Pinner                              Example                                                                            feet/minute                                                                         kilovolts                                                                            milliamperes                                                                         gauge                                                                              inch   inches                                                                             bubbles                             __________________________________________________________________________    F    138   5      2.85   --   0.5    3.74 None                                3    138   5      1.6    0.6  0.75   1.75 None                                4    122   5      1.33   0.6  0.75   1.75 None                                5    132   5      1.4    0.6  0.75   1.75 None                                __________________________________________________________________________

EXAMPLES 6, 7 AND 8

Molten polyethylene terephthalate film was cast and quenched byelectrostatic and suction means using the apparatus shown in FIGS. 6 and7 of the drawings. The suction knife employed had side orifices for thecontrol of air passage across the edges of the cast film. The castingand quenching conditions and the characteristics of the resulting filmare shown in Table 4.

                                      TABLE 4                                     __________________________________________________________________________                             Suction at                                                                          Suction at                                                                          Extrusion                                           Pinning       main orifice                                                                        side orifices                                                                       die to                                        Casting                                                                             wire   Pinning wire                                                                         of air knife                                                                        of air knife                                                                        casting                                       drum speed                                                                          potential                                                                            current                                                                              inch of                                                                             inch of                                                                             drum gap                                                                           Neck-in                                                                            Pinner                         Example                                                                            feet/minute                                                                         kilovolts                                                                            milliamperes                                                                         water gauge                                                                         water gauge                                                                         inch inches                                                                             bubbles                        __________________________________________________________________________    6    124   5      1.55   0.5   1.5   0.44 1.25 None                           7    138   5      1.7    0.5   1.5   0.44 1.25 None                           8    152   5      1.9    0.5   1.5   0.44 1.25 None                           __________________________________________________________________________

We claim:
 1. A process for extruding and quenching a polymeric film,which comprises extruding a molten polymeric film on to an electricallyearthed moving casting surface, said casting surface moving at a linearspeed greater than the speed at which bubbles can be prevented frombeing entrapped between the molten polymeric film and the castingsurface when electrostatic pinning is employed alone, depositingelectrostatic charges upon part or the whole of the molten film width bymeans of an electrostatic pinning electrode supported in spaced relationto the molten film in the proximity of or prior to the region of firstcontact of the film with the casting surface, and applying suction tothe space bounded by the path of the molten film prior to its firstcontact with the casting surface and that part of the moving castingsurface prior to the region of first contact with the molten film,shortening the path of the molten film, the deposited electrostaticcharges and the suction operating in combination to adhere the moltenfilm to the casting surface, and to reduce the effect of neck-in.
 2. Aprocess according to claim 1, in which the film is a film ofpolyethylene terephthalate.
 3. A process according to claim 1, in whichthe casting surface is a cylindrical drum rotatable at speeds exceeding150 feet/minute.
 4. A process according to claim 1, in which the suctionis provided by a suction knife.
 5. A process according to claim 4, inwhich the suction knife has a slot orifice located between 0.06 and 3.5inches from the molten web and between 0.06 and 3.0 inches from thecasting surface.
 6. A process according to claim 1, in which the suctionapplied is in the range from 0.1 to 3.0 inches of water.
 7. A processaccording to claim 1, in which suction or a partial vacuum is appliedsubstantially in the vicinity of the molten film edges and substantiallytransversely to the direction of movement of the casting surface.
 8. Aprocess according to claim 7, in which the transverse suction or partialvacuum is greater in magnitude than that applied between the movingcasting surface and the molten film.